Color temperature is a characteristic of visible light that has important applications in lighting. The color temperature of a light source is the temperature of an ideal black-body radiator that radiates light of comparable hue to that of the light source. Color temperature is conventionally stated in the unit of absolute temperature, the kelvin, having the unit symbol K.
Typically, the color temperature of a white light source is determined predominantly by the mechanism used to generate the light. For example incandescent light sources typically have a relatively low color temperature around 3000K, called “warm white”. Conversely, fluorescent lights have a higher color temperature around 7000K, called “cool white”. The choice of warm or cool white is determined when purchasing the light source or when a building design or construction is completed. In many situations, such as street lighting, warm white and cool white light are used together.
Recently, white light emitting LEDs (“white LEDs”) have become more popular and are rapidly being used to replace conventional fluorescent, compact fluorescent and incandescent light sources. White LEDs generally include one or more photoluminescent materials (e.g., one or more phosphor materials), which absorb a portion of the radiation emitted by the LED and re-emit light of a different color (wavelength). The phosphor material may be provided as a layer on, or incorporated within a wavelength conversion component that is located remotely from the LED. Typically, the LED generates blue light and the phosphor(s) absorbs a percentage of the blue light and re-emits yellow light or a combination of green and red light, green and yellow light, green and orange or yellow and red light. The portion of the blue light generated by the LED that is not absorbed by the phosphor material combined with the light emitted by the phosphor provides light which appears to the eye as being nearly white in color. Such white light LEDs are characterized by their long operating life expectancy (>50,000 hours) and high luminous efficacy (70 lumens per watt and higher).
For such white LEDs, light is emitted by photoluminescence rather than thermal radiation. Thus, the emitted radiation does not follow the form of a black-body spectrum. These sources are assigned what is known as a correlated color temperature (CCT). CCT is the color temperature of a black body radiator which to human color perception most closely matches the light from the lamp.
One concern with many white LED configurations is that they may generate light having inconsistent color emissions. For example, consider the lighting configuration shown in FIG. 1 which includes a blue LED light source 104 and a remote phosphor component 102. It can be seen that the remote phosphor component 102 has a tall and narrow aspect ratio, such that there are unequal distances from the LED 104 to different portions of the remote phosphor component 102. In operation, blue excitation light interacts with the phosphor material in component 102 to generate photoluminescence light. However, differing portions of the component 102 may receive differing levels of blue light and may also interact to different extents. This is due to many possible reasons. One possible reason is that, given the tall and narrow profile of the component 102, there exists many different distances for the conversion paths to convert the blue LED light into phosphor light. In addition, the angles at which the blue light is emitted from LED 104 causes at least some of the blue light to strike the inner surface of the component at angles above the critical angle thereby reflecting such light upwardly towards the top of the component 102 rather than coupling into the component and interacting with the phosphor. In contrast, at locations closest to the LED 104 blue light strikes the component at angles equal to or less than the critical angle thereby coupling into the component and interacting with the phosphor to generate converted light. Additionally differences in conversion path distances (i.e. the effective distance that light can travel through the component) can cause a shift in color over the length of the component 102.
Moreover, differing levels of emitted phosphor light from the component 102 contribute to shifts in color. Due to the isotropic nature of the photoluminescence process, photoluminescence light is emitted equally in all directions, resulting in converted light re-entering the interior of the component. In this way, light which is converted at one location on the component 102 may be “recycled” before being emitted from the lighting device at a different location on the component.
These problems potentially exist with any LED-based lighting architecture that may include long light paths from an excitation light source to a remote phosphor component. FIG. 2 illustrates how a light pipe 202 having side emission chambers 206 may generate shifts in color over the length of the component due to long light paths. Color shifts occur in the light pipe 202 due to excitation light not being evenly distributed from the LED source 204 due to the length of the light pipe. The unequal conversion paths may also be caused by indirect light paths from the LED 204 caused by reflective light off reflective surfaces 214 within the light pipe 202. The interior of the light pipe 202 may be filed with either air or an optical medium.
FIG. 3 illustrates a planar remote phosphor component 302 mounted over a wide mixing chamber 306, where the configuration includes a central excitation source 304. Color shifts occur due to excitation light not being evenly distributed from the center-only source 304 across the width of the planar phosphor component 302, both due to unequal length light paths directly from the LED 304 to the component 302, as well as unequal length reflective light paths caused by reflection off reflective surfaces 314 within the mixing chamber 306.
This problem is most visible in high aspect ratio configurations. Uniform components (such as domes and spheres) generally do not have this issue.
In many cases, visible inconsistencies in color over a lighting product can detrimentally affect the aesthetic appearance and usefulness of that product. Therefore, it is important for a light manufacturer to be able to construct a lamp having a consistent, advertised color.